9
Building blocks: Router (Switch) 9 Router: receives and forwards packets Buffers: – Queuing – Decouple the allocation of adjacent channels in time – Can be organized as virtual channels.

10
10 Building blocks: Links Connects two routers in both directions on a number of wires (e.g., 32 bits) In addition, wires for control are part of the link too Can be pipelined (include handshaking for asynchronous)

12
NoC topologies 12 The topology is the network of streets, the roadmap.

13
Direct topologies Direct Topologies – Each node has direct point-to-point link to a subset of other nodes in the system called neighboring nodes – As the number of nodes in the system increases, the total available communication bandwidth also increases – Fundamental trade-off is between connectivity and cost Most direct network topologies have an orthogonal implementation, where nodes can be arranged in an n-dimensional orthogonal space – e.g. n-dimensional mesh, torus, folded torus, hypercube, and octagon 13

14
2D-mesh It is most popular topology All links have the same length – eases physical design Area grows linearly with the number of nodes Must be designed in such a way as to avoid traffic accumulating in the center of the mesh 14

15
Torus Torus topology, also called a k-ary n-cube, is an n-dimensional grid with k nodes in each dimension k-ary 1-cube (1-D torus) is essentially a ring network with k nodes – limited scalability as performance decreases when more nodes k-ary 2-cube (i.e., 2-D torus) topology is similar to a regular mesh – except that nodes at the edges are connected to switches at the opposite edge via wrap-around channels – long end-around connections can, however, lead to excessive delays 15

16
Folding torus Folding torus topology overcomes the long link limitation of a 2-D torus links have the same size Meshes and tori can be extended by adding bypass links to increase performance at the cost of higher area 16

17
Octagon Octagon topology is another example of a direct network – messages being sent between any 2 nodes require at most two hops – more octagons can be tiled together to accommodate larger designs by using one of the nodes as a bridge node 17

18
Indirect topologies Indirect Topologies – each node is connected to an external switch, and switches have point-to-point links to other switches – switches do not perform any information processing, and correspondingly nodes do not perform any packet switching – e.g. SPIN, crossbar topologies Fat tree topology – nodes are connected only to the leaves of the tree – more links near root, where bandwidth requirements are higher 18

22
Routing algorithms Routing is the route/path (a sequence of channels) of streets from source to destination. The routing method steers the car. Routing determines the path followed by a message through the network to its final destination. Responsible for correctly and efficiently routing packets or circuits from the source to the destination – Path selection between a source and a destination node in a particular topology Ensure load balancing Latency minimization Flexibility w.r.t. faults in the network Deadlock and livelock free solutions Routing schemes/techniques/algos can be classified/looked-at as: – Static or dynamic routing – Distributed or source routing – Minimal or non-minimal routing 22

23
Static/deterministic vs. Dynamic/adaptive Routing Static routing: fixed paths are used to transfer data between a particular source and destination – does not take into account current state of the network advantages of static routing: – easy to implement, since very little additional router logic is required – in-order packet delivery if single path is used Dynamic/adaptive routing: routing decisions are made according to the current state of the network – considering factors such as availability and load on links path between source and destination may change over time – as traffic conditions and requirements of the application change more resources needed to monitor state of the network and dynamically change routing paths able to better distribute traffic in a network 23

24
Example: Dimension-order Routing Static XY routing (commonly used): – a deadlock-free shortest path routing which routes packets in the X-dimension first and then in the Y-dimension Used for tori and mesh topologies Destination address expressed as absolute coordinates It may introduce imbalance low bandwidth x +y For torus, a preferred direction may have to be selected. For mesh, the preferred direction is the only valid direction.

25
Example: Dynamic Routing A locally optimum decision may lead to a globally sub-optimal route To avoid slight congestion in (01-02), packets then incur more congested links

26
Routing mechanics: Distributed vs. Source Routing Routing mechanics refers to the mechanism used to implement any routing algorithm. Distributed routing: each packet carries the destination address – e.g. XY co-ordinates or number identifying destination node/router – routing decisions are made in each router by looking up the destination addresses in a routing table or by executing a hardware function Source routing: packet carries routing information – pre-computed routing tables are stored at NI – routing information is looked up at the source NI and routing information is added to the header of the packet (increasing packet size) – when a packet arrives at a router, the routing information is extracted from the routing field in the packet header – does not require a destination address in a packet, any intermediate routing tables, or functions needed to calculate the route 26

27
Minimal vs. Non-minimal Routing Minimal routing: length of the routing path from the source to the destination is the shortest possible length between the two nodes – source does not start sending a packet if minimal path is not available Non-minimal routing: can use longer paths if a minimal path not available – by allowing non-minimal paths, the number of alternative paths is increased, which can be useful for avoiding congestion – disadvantage: overhead of additional power consumption Minimal adaptive routing is unable to avoid congested links in the absence of minimal path diversity

28
28 No winner routing algorithm

29
Routing Algorithm Requirements Routing algorithm must ensure freedom from deadlocks – Deadlock: occurs when a group of agents, usually packets, are unable to progress because they are waiting on one another to release resources (usually buffers and channels). – common in WH switching – e.g. cyclic dependency shown below – freedom from deadlocks can be ensured by allocating additional hardware resources or imposing restrictions on the routing – usually dependency graph of the shared network resources is built and analyzed either statically or dynamically 29

30
Routing Algorithm Requirements Routing algorithm must ensure freedom from livelocks – livelocks are similar to deadlocks, except that states of the resources involved constantly change with regard to one another, without making any progress – occurs especially when dynamic (adaptive) routing is used – e.g. can occur in a deflective hot potato routing if a packet is bounced around over and over again between routers and never reaches its destination – livelocks can be avoided with simple priority rules Routing algorithm must ensure freedom from starvation – under scenarios where certain packets are prioritized during routing, some of the low priority packets never reach their intended destination – can be avoided by using a fair routing algorithm, or reserving some bandwidth for low priority data packets 30

32
32 Switching strategies Switching establishes the type of connection between source and destination. It is tightly coupled to routing. Can be seen as a flow control mechanism as a problem of resource allocation. Allocation of network resources (bandwidth, buffer capacity, etc.) to information flows – phit is a unit of data that is transferred on a link in a single cycle – typically, phit size = flit size Two main switching schemes: 1.circuit (or path) switching 2.packet switching

33
33 1. Pure Circuit Switching It is a form of bufferless flow control. Advantage: Easier to make latency guarantees (after circuit reservation) Disadvantage: does not scale well with NoC size – several links are occupied for the duration of the transmitted data, even when no data is being transmitted Circuit set-up Two traversals – latency overhead Waste of bandwidth Request packet can be buffered Circuit utilization Third traversal – latency overhead Contention-free transmission Poor resource utilization

35
35 2. Packet Switching It is a form of buffered flow control Packets are transmitted from source and make their way independently to receiver – possibly along different routes and with different delays Zero start up time, followed by a variable delay due to contention in routers along packet path – QoS guarantees are harder to make

36
36 Three main packet switching scheme variants 1. Store and Forward (SAF) switching – packet is sent from one router to the next only if the receiving router has buffer space for entire packet – buffer size in the router is at least equal to the size of a packet – Disadvantage: excessive buffer requirements 2. Virtual Cut Through (VCT) switching – forwards first flit of a packet as soon as space for the entire packet is available in the next router – reduces router latency over SAF switching – same buffering requirements as SAF switching 3. Wormhole (WH) switching – flit is forwarded to receiving router if space exists for that flit (1)After A receives a flit of the packet, A asks B if B is ready to receive a flit (2)B A, ack (3)A sends a flit to B. A B Pipelining on a flit (flow control unit) basis flit size < packet size Smaller data space is needed than store-and-forward

37
37 Wormhole Switching Issues Wormhole switching suffers from packet blocking problems An idle channel cannot be used because it is owned by a blocked packet… – Although another packet could use it! Using virtual channels helps address this A X Idle Blocked B 2 virtual channels A B Wormhole

39
Flow control Flow control dictates which messages get access to particular network resources over time. It manages the allocation of resources to packets as they progress along their route. It controls the traffic lights: when a car can advance or when it must pull off into a parking lot to allow other cars to pass. Can be viewed as either a problem of resource allocation (switching strategy) or/and one of contention resolution. Recover from transmission errors Commonly used schemes: – STALL-GO flow control – ACK-NACK flow control – Credit based flow control 39 A BC Block Buffer full Dont send Buffer full Dont send Backpressure

40
STALL/GO low overhead scheme requires only two control wires – one going forward and signaling data availability – the other going backward and signaling either a condition of buffers filled (STALL) or of buffers free (GO) can be implemented with distributed buffering (pipelining) along link good performance – fast recovery from congestion does not have any provision for fault handling – higher level protocols responsible for handling flit interruption 40

41
ACK/NACK when flits are sent on a link, a local copy is kept in a buffer by sender when ACK received by sender, it deletes copy of flit from its local buffer when NACK is received, sender rewinds its output queue and starts resending flits, starting from the corrupted one implemented either end-to-end or switch-to-switch sender needs to have a buffer of size 2N + k – N is number of buffers encountered between source and destination – k depends on latency of logic at the sender and receiver fault handling support comes at cost of greater power, area overhead 41

44
Clocking schemes Fully synchronous – single global clock is distributed to synchronize entire chip – hard to achieve in practice, due to process variations and clock skew Mesochronous – local clocks are derived from a global clock – not sensitive to clock skew – phase between clock signals in different modules may differ – deterministic for regular topologies (e.g. mesh) – non-deterministic for irregular topologies – synchronizers needed between clock domains Pleisochronous – clock signals are produced locally Asynchronous – clocks do not have to be present at all 44

46
Quality of Service (QoS) QoS refers to the level of commitment for packet delivery – refers to bounds on performance (bandwidth, delay, and jitter=packet delay variation) Two basic categories – Best effort (BE) only correctness and completion of communication is guaranteed usually packet switched worst case times cannot be guaranteed – Guaranteed service (GS) makes a tangible guarantee on performance, in addition to basic guarantees of correctness and completion for communication usually (virtual) circuit switched 46

50
Examples Æthereal – Developed by Philips – Synchronous indirect network – WH switching. Contention-free source routing based on TDM – GT as well as BE QoS. GT slots can be allocated statically at initialization phase, or dynamically at runtime – BE traffic makes use of non-reserved slots, and any unused reserved slots also used to program GT slots of the routers – Link-to-link credit-based flow control scheme between BE buffers to avoid loss of flits due to buffer overflow HERMES – Developed at the Faculdade de Informática PUCRS, Brazil – Direct network. 2-D mesh topology – WH switching with minimal XY routing algorithm – 8 bit flit size; first 2 flits of packet contain header – Header has target address and number of flits in the packet – Parameterizable input queuing to reduce the number of switches affected by a blocked packet – Connectionless: cannot provide any form of bandwidth or latency GS 50

51
51 Examples MANGO – Developed at the Technical University of Denmark – Message-passing Asynchronous Network-on-chip providing GS over open core protocol (OCP) interfaces – Clockless NoC that provides BE as well as GS services – NIs (or adapters) convert between the synchronous OCP domain and asynchronous domain – Routers allocate separate physical buffers for VCs for simplicity, when ensuring GS – BE connections are source routed BE router uses credit-based buffers to handle flow control length of a BE path is limited to five hops – Static scheduler gives link access to higher priority channels admission controller ensures low priority channels do not starve Nostrum – Developed at KTH in Stockholm – 2-D mesh topology. SAF switching with hot potato (or deflective) routing – Support for switch/router load distribution, guaranteed bandwidth (GB), multicasting – GB is realized using looped containers implemented by VCs using a TDM mechanism container is a special type of packet which loops around VC multicast: simply have container loop around on VC having recipients – Switch load distribution requires each switch to indicate its current load by sending a stress value to its neighbors

52
52 Examples Octagon – Developed by STMicroelectronics – Direct network with an octagonal topology – 8 nodes and 12 bidirectional links. Any node can reach any other node with a max of 2 hops – Can operate in packet switched or circuit switched mode – Nodes route a packet in packet switched mode according to its destination field node calculates a relative address and then packet is routed either left, right, across, or into the node – Can be scaled if more than 8 nodes are required: Spidergon QNoC – Developed at Technion in Israel – Direct network with an irregular mesh topology. WH switching with an XY minimal routing scheme – Link-to-link credit-based flow control – Traffic is divided into four different service classes signaling, real-time, read/write, and block-transfer signaling has highest priority and block transfers lowest priority every service level has its own small buffer (few flits) at switch input – Packet forwarding is interleaved according to QoS rules high priority packets able to preempt low priority packets – Hard guarantees not possible due to absence of circuit switching Instead statistical guarantees are provided

54
54 Examples Xpipes – Developed by the Univ. of Bologna and Stanford University – Source-based routing, WH switching – Supports OCP standard for interfacing nodes with NoC – Supports design of heterogeneous, customized (possibly irregular) network topologies – Go-back-N retransmission strategy for link level error control errors detected by a CRC (cycle redundancy check) block running concurrently with the switch operation – XpipesCompiler and NetChip compilers tools to tune parameters such as flit size, address space of cores, max. number of hops between any two network nodes, etc. generate various topologies such as mesh, torus, hypercube, Clos, and butterfly CHAIN (Silistix who did not survive?) – Developed at the University of Manchester – Implemented entirely using asynchronous circuit techniques exploit low power capabilities – Targeted for heterogeneous low power systems, in which the network is system specific – It makes use of 1-of-4 encoding, and source routes BE packets – It has been implemented in smart cards – Recent work from the group involved with CHAIN concerns prioritization in asynchronous networks

84
Problem formulation Given – An application (or a set of concurrent applications) already mapped and scheduled into a set of IPs – A network topology Find the best mapping and the best routing function which – Maximize Performance (Minimize the mapping coefficient) – Maximize fault tolerant characteristics (Maximize the robustness index) Such that – The aggregated communications assigned to any channel do not exceed its capacity 84